The present invention relates to inductive sensors and, more particularly, to an inductive sensor that determines the identity and position of a conductive pattern, or object, in proximity to the sensor, and that is additionally substantially temperature and component tolerance independent.
A sensor that determines the identity and position of a conductive pattern or object is generally known as a xe2x80x9cpresence sensorxe2x80x9d. Most presence sensors use reed switches and magnets to track the motion of magnetic elements. These devices usually have one reed switch placed under a plurality of discrete positions on the sensor. When an object with a magnet integrated therein is placed in a particular position, the reed switch is activated and remains closed until the object is removed. In this way, the position of the objects placed on the sensor may be determined and tracked.
However, such devices cannot determine the identities of the objects placed upon the surface of the sensor. Only when specific, predefined objects are placed in predefined positions on the sensory surface can such devices determine the identities of the objects placed upon the sensor. Furthermore, such devices are often inaccurate if the objects are moved quickly, or if two objects occupy the same position on a sensory surface.
In moving away from a standard reed switch/magnetic element presence sensor, presence sensors utilizing a change in the inductance of sense coils to indicate the location and/or identity of an object have been developed. Such a presence sensor may be found in U.S. Pat. No. 5,791,648 (hereafter, the ""648 patent), which is entitled xe2x80x9cInductive Sensory Apparatusxe2x80x9d and is hereby incorporated by reference. The inductive sensory apparatus of the ""648 patent utilizes a matrix of coils embedded in the surface of the apparatus. A multiplexor selects the individual coils by connecting them sequentially to an LC oscillator where they act as the inductors in the oscillator circuit. Changes in the inductance of any coil results in corresponding changes in the amplitude and the frequency of oscillation of the oscillator circuit. The oscillation amplitude of each coil is compared against at least one voltage threshold by an amplitude comparator. The amplitude comparator is sampled by a microcontroller to detect the inductance of each coil. The microcontroller then assigns a value to each coil that is determined by the inductance of the coil. By assigning a value to each individual coil, the microcontroller determines the code that uniquely identifies the object placed next to the inductive sensor apparatus.
While the inductive sensory apparatus of the ""648 patent is a significant improvement over a standard reed switch/magnetic element presence sensor, it has its own limitations. Specifically, the LC oscillator of the inductive sensory apparatus, which is used to detect changes in the self inductance of the sensory coils near a conductive pattern, is subject to amplitude and frequency variations due to changes in temperature and component tolerances. This results in a shift in the quiescent operating amplitude of the oscillator. The shift in the quiescent operating amplitude limits the ability of the inductive sensory apparatus to detect small variations in amplitude, caused by the proximity of conductive material to the inductive sensory apparatus.
Still another limitation of the inductive sensory apparatus of the ""648 patent is that it detects only the presence or absence of conductive material in close proximity to each of the sense coils, which limits the number of objects that can be detected with a given number of sense coils, due to the use of binary coded conductive patterns. Yet another limitation of the inductive sensory apparatus of the ""648 patent is that while it can detect the presence or discrete location of a conductive object, it cannot detect the exact position or displacement of such an object.
In view of the above, there is a need for an electronic inductive sensory apparatus that incorporates improved sensitivity to detect objects encoded with a conductive pattern, or to detect the presence or location of a conductive object at greater sensing distances for a given coil size. Further, there is a need for an electronic inductive sensory apparatus that has improved tolerance and stability by automatically compensating for variations in temperature and component tolerances. Further still, there is a need for an electronic inductive sensory apparatus that can detect an increased number of objects with a given number of sense coils, through the detection of multiple conductive states. There is also a need for an electronic inductive sensory apparatus that can detect the position or displacement of an object.
The needs described above are in large measure met by the inductive sensory system of the present invention. The inductive sensory system generally comprises a sense inductor, a reference sense inductor, an oscillator, a feedback control loop, and a comparator. The oscillator is connectable to both the sense inductor and the reference sense inductor. Upon the oscillator being connected to the reference sense inductor, the feedback control loop establishes a fixed reference level. Upon the sense inductor being connected to the oscillator, the comparator compares the state of the sense inductor against a threshold that is derived from the fixed reference level to determine the change in inductance of the sense inductor.
The inductive sensory system further includes a processor that selects between the sense inductor and the reference sense inductor. The feedback control loop maintains the fixed reference level substantially independently of ambient temperature variations, supply voltage variations, and component tolerance variations. The sense inductor may be of various configurations including a flat, round or square sense coil and a triangularly-shaped or rectangularly-shaped sense coil. The sense inductors may be incorporated into a gameboard, doll or book, or a template positioned beneath a book. to detect conductive spots within pages of the book.
The inductive sensory system may be configured for detection of multiple conductive states. In doing so, the reference level established by the feedback control loop is used to derive at least two thresholds for comparison. The different thresholds allow for detection of different conductive levels in an object. These different conductive levels may be detected by the system regardless of the orientation of the object at a given sensing distance. The objects are preferably encoded with a printed pattern or different materials that have different conductivities. Specifically, the inductive sensory system may be used to detect trinary patterns of objects encoded with three distinct conductive states, e.g., covered, uncovered, partially covered with conductive material.
The inductive sensory system may employ triangular or rectangular shaped coils to create a position sensor that may be incorporated into an attenuator or slide controller. As conductive material is moved along the coil axis a voltage is generated that is linearly proportional to the displacement of the conductive object. This voltage or its digital representation may be used to control a variety of mechanical devices, audio volume, temperature, etc.